Method and device for determining the centric position of human teeth

ABSTRACT

The invention concerns methods and a device for the determination of the centric position of a human set of teeth with improved precision. This is needed for the adjustment of an articulator in the production of dentures. For this purpose a plurality of successive bite movements or displacement movements of the lower jaw are recorded in form of electronic signal data and evaluated for the determination of the centric position.

The invention finds application in the dental field, in particular inthe production of dentures.

The most common technique for the production of dentures is thefollowing. The dentist takes imprints of the teeth of the upper jaw andthe lower jaw of the patient. Plaster models of the upper jaw and thelower jaw are produced from the imprints. These are placed in anarticulator which consists of two parts which are connected with eachother by two joints and which simulate the human set of teeth. The twojoints correspond to the human jaw joints. After putting the plastermodels in place the joints must be adjusted patient specifically.

A known method for the determination of the centric position consists inthat a plate-like plastically deformable register is inserted betweenthe opened teeth of the patient and the patient is then caused to bite.Through this imprints of the teeth of the upper jaw and the lower jaware formed in the register. After hardening, the register is placedbetween the plaster models of the upper and lower jaw emplaced in aregistry. Then, the lower jaw is displaced relative to the upper jaw, byadjustment of the two joints of the articulator representing the jawjoints, for so long that the teeth of the plaster models of the upperand lower jaw engage into the imprints on the two sides of the registerwith fitting seating.

Another likewise known method consists in that in each case a plate isfastened, in the mouth of the patient, to the teeth of the upper jaw andthe lower jaw by means of a plastically deformable mass. The two platesextend through the oral cavity. The plate fastened to the teeth of thelower jaw carries a supporting pin rising up in the middle. The platefastened to the teeth of the upper jaw has a wax layer at its undersidein which the supporting pin, upon impingement, produces an imprint. Theimprint defines the centric position. The two plates are then removedfrom the mouth of the patient and put onto the teeth of the plastermodels in the articulator. The joints of the articulator are then sodisplaced that the supporting pin again dips into the imprint of the waxlayer at the underside of the upper plate. With that, the articulator isadjusted to the centric position. Thus, here, the centric position is areference position fixed in the patient mouth and in employed in thearticulator for adjustment.

A third known method for the determination of the centric position is avariation of the second method just described. Here, the patient is notonly caused to bite once, but rather his lower jaw is to carry out afore and back movement, as well as side movements to two sides. Thereby,a T-like trace arises in the wax layer of the upper plate, wherein theupper T-limb is upwardly arched. One calls this arching a Gothic arch.The intersection point of the two T-limbs is now selected as that pointwith which the support pin is lined up in the articulator for adjustingthe centric position.

There is also known a device sold by the applicant under the trade name“Digma” or “ARCUS-Digma”. This device is an aid for the setting ofarticulators. For this purpose, it carries out an electronic positionanalysis of the lower jaw. To measure the movements of the lower jawwith respect to the upper jaw in each case there is connected with theupper jaw and the lower jaw a carrier frame. The carrier frame connectedto the upper jaw at the upper part of the head has four microphones; thecarrier frame connected to the lower jaw has three ultrasoundtransmitters. The twelve paths between the three transmitters and thefour microphones are evaluated with regard to their length changes uponmovement of the lower jaw. Up to now, the device has not found anyapplication for the determination of the centric position.

The invention addresses the task of solving the problem described below.If a patient moves the lower jaw with respect to the upper jaw and theteeth meet each other, then this does not mean inevitably that the upperjaw always takes up the same position with respect to the lower jaw. Thementioned position rather differs in dependence upon the tension of thechewing muscles, upon signals issued by the teeth when they meet oneanother, upon of the pressure on the menisci in the jaw joints and uponvarious signals from the brain. The latter are influenced by whether thepatient is tired or is under stress. The previous methods for thedetermination of the centric position are therefore inaccurate and thisbecause the single bite or single movements is or are subject to randomfactors. The invention is based on the object of improving the precisionof the method for the determination of the centric position.

Starting out from the second known method for the determination of thecentric position a first solution in accordance with the inventionconsists in accordance with claim 1 in that a plurality of bitingmovements are carried out, in that the biting movements are detectedusing measurement technology and converted into electrical signal data,and in that from the signal data of a plurality of end positions thecentric position is determined.

The centric position can be determined for example by averaging of thesignal data of the plurality of end positions in a coordinate plane.

Another possibility consists in that for the determination of thecentric position only the signal data items, in one coordinate plane,lying in a restricted hit field are evaluated.

Starting out from the third known method for the determination of thecentric position of a human set of teeth, described above, in which thelower jaw is caused to carry out at least one movement with respect tothe upper jaw and in which at least two movements are evaluated for thedetermination of the centric position, a further proposal in accordancewith the invention for solution of the problem consists in that thelower jaw in the bite end position is caused to carry out a plurality offirst displacement movements running between the forward and therearward end position as well as a plurality of second displacementmovements running between the two lateral end positions, in that thefirst and second displacement movements are detected using measurementtechnology and transformed into electrical signal data, and in that thesignal data is evaluated to the effect that the intersection point ofthe first and the second displacement movements is determined as centricposition. The signal data of the first and the second displacementmovement as well as the signal data of the determined centric positioncan be represented on a display as symbols, such as e.g. points. Therepresentation can be effected in a coordinate plane, in which thecoordinates are Cartesian coordinates.

A certain centric position need not necessarily be coupled with certainpositions of the two jaw joints. Due to asymmetric biting by the patienta certain centric position can also correspond to different pairs of jawjoint positions. If one wishes to remove this uncertainty, furtherinformation must be available for the adjustment of the articulator.This is for example the case if the centric position is used to program(introduction of tooth imprints) a register. For this purpose, thepatient—possibly with the guiding support of the dentist—moves his lowerjaw of into the determined centric position, so that the teeth of upperand lower jaw produce imprints in a plastic register placed betweenthem. The imprints contain the additional information about the jawjoint positions. The register then can be used, in accordance with theknown method described first, in the articulator to adjust the jointsthereof.

Alternatively to this it is possible, alongside or instead of the signaldata for the one centric position, to determine the corresponding signaldata for a mean position of the two jaw joints. If this data isavailable, the use of a register can be foregone. The mean position ofthe two jaw joints can then be used directly for adjustment of the twojoints of the articulator, by either hand or by motorized means.

A development of the first proposal for a solution can consist in thatthe signal data of the bite end positions of all biting movements andthe signal data of the determined centric position are represented on adisplay as symbols, such as points for example. The representation canbe effected in a coordinate plane which allows the actual spacings ofthe bite end positions from the centric position to be recognized incorresponding measurement units. Preferably, Cartesian coordinates areused for this.

An expedient development of the second proposal for a solution inaccordance with the invention can consist in that the signal data of thejaw joint positions corresponding to the centric position arerepresented on a display as symbols, such as points for example, in eachcase in two coordinate planes, and in that the coordinate planes includean angle, preferably an angle of 90°. The one coordinate plane mayrepresent for example that sectional plane through the patient headwhich runs on the one hand between above (cranial) and below (cardial)and on the other hand in front (ventral) and behind (dorsal), and inthat the other coordinate plane represents that sectional plane throughthe patient head which on the one hand runs between in front (ventral)and behind (dorsal) and on the other hand between the body axis (medial)and the appropriate side (lateral). The coordinates may be representedin the manner of polar coordinates in the form of concentric rings.Thereby, the symbols for the jaw joint positions which correspond to thecentric position, with freedom from pain of the jaw joints, can bedisplaced into the centre of the polar coordinates as a reference forall other possible jaw joint positions.

The device for carrying out one of the above-described methods for thedetermination of the centric position of a human set of teeth may have,like the known Digma device, carrier frames for sender and receiverelements, wherein in each case one carrier frame is connectable in areproducible position with the lower jaw and the upper jaw of thepatient in question and wherein the distances between the transmitterand receiver elements are measured and evaluated for obtaining signaldata.

Thereby, it has proved that the precision provided with the four senderelements and three receiver elements of the known Digma device isinsufficient to ensure the desired precision for the determination ofthe centric position. The device in accordance with the invention istherefore different from the known Digma device in that, among otherthings, more than three sender elements and more than four receiverelements are provided. Further relevant differences arise from thecorresponding means with which the individual method steps are to berealized. These means arise from the method steps (e.g. means for thecalculation of the mean value from the signal data etc.).

The invention will be explained below with reference to the drawings.

FIG. 1 shows an articulator with plaster models and register in place;

FIG. 2 shows the head of a patient with in-place carrier elements forthe sender and sensor elements;

FIG. 3 shows a device for centric determination;

FIG. 4 shows a display with different hits for the selection of thecentric position;

FIG. 5 shows a display as in FIG. 4 additionally with four polarcoordinate planes for the indication of the tested jaw joint positions;

FIG. 6 shows a display as in FIG. 4 in the case of which, however,another method for centric determination was selected.

The articulator 1 shown in FIG. 1 is conventional and described e.g. inDE 42 11 020 C2. It consists of two yokes 2, 3 which are connected witheach other by joints 4, 5 of which only one is visible here. The joints4, 5 are adjustable for setting the centric position. The spacing of theyokes is regulatable with a supporting pin 6. The plaster model 7 of thelower jaw of a patient is attached to the yoke 2. The plaster model 8 ofthe upper jaw of the patient is attached to the yoke 3. A register 9, ofa curable, initially plastically deformable material, is located betweenthe teeth of the two plaster models 7, 8.

For the making of the two plaster models 7, 8, at first the dentist musttake imprints of the teeth of the upper jaw and the lower jaw. For thishe uses shells adapted to the tooth row form, in which a plasticallydeformable mass is located, which the dentist presses onto the teeth.Liquid plaster is then cast into the mould provided in this way, whichafter hardening yields plaster models of the upper jaw and of the lowerjaw of the patient. Then, with the conventional method for thedetermination of the centric position, the patient is caused by thedentist to move the lower jaw against the upper jaw in the form of asingle biting movement, so that imprints of the teeth of the upper jawand the lower jaw are formed in the register 9. The substrate 9 is then,after hardening, placed in the articulator between the teeth of the twoplaster models 7, 8 as illustrated in FIG. 1. To ensure that thearticulator simulates as far as possible the parameters of the set ofteeth of the patient, now the joints 4, 5 of the articulator and thedistance pin 6 are so adjusted that the teeth of the two plaster models7, 8 engage into the imprints of the register 9 in form-fitting manner.When this adjustment is complete, the plaster model can be processed ina conventional manner for the production of dentures.

However, it has proved that the previous method of determination of thecentric position is inaccurate because the single bite is affected byrandom factors, for example affected by the state of stress of thepatient, by the sensitiveness of his teeth when meeting thecounter-teeth, as well as by the condition of the menisci in the jawjoints.

Thus, to increase the precision in the determination of the centricposition on the one hand there is used an electronic measurementprocedure known in itself, with which previously the movement of thelower jaw with respect to the upper jaw has been determined in threedimensions. In other words with this method electronic signal data isproduced and evaluated for representation of the movements of the lowerjaw. Alongside the application of the known electronic measurementprocedure, a statistical evaluation of a plurality of movements of thelower jaw with respect to the upper jaw is carried out.

The new measuring arrangement for the determination of the centricposition is shown in FIG. 2. On the upper part of the head 11 of apatient 10 there is attached a carrier frame 12 which has four arms.Each arm carries two downwardly directed ultrasound microphones asreceiver elements. A further carrier frame 15 is connected with thelower jaw 14 of the patient 10. This is adhered by means of a bite forkon the teeth of the lower jaw with the use of a plastic mass. The frame15 carries four upwardly directed ultrasound transmitters as senderelements.

In the case of the known Digma device one works with three senderelements and four receiver elements. This yields twelve measurementsections, length changes of which are evaluated upon movement of thelower jaw. It has proved, however, that the thus obtainable measuringprecision is not sufficient to determine the centric position with thenow desired precision. The precision is, however, achieved with the newdevice by the eight sender elements and four receiver elements provided,with which 32 measurement sections can be evaluated. It is, however,emphasized at this point that the number of the sender elements and thereceiver elements has effect only with regard to the precision, but isnot decisive for the fundamental function of the methods in accordancewith the invention.

The new device 19 for carrying out the methods in accordance with theinvention is shown in FIG. 3. It is docked on the arm 18 of a dentaltreatment unit 17 and has a display 20. The device 19 is connected viaconnection cable or in cable-less manner with the sender elements 16 andthe receiver elements 13 on the carrier frames 12 and 15 in FIG. 2. Itmeasures the bite end positions of the lower jaw 14 in the form ofsignal data.

The bite end positions of a plurality of successive biting movements ofthe patient are illustrated on the display 20, illustrated in FIG. 4 toan enlarged scale, in the form of points 21. The display is covered witha grid of Cartesian coordinates. One recognizes that the bite endpositions differ more or less from each other. The points 21 lyingwithin a hit field indicated by a circle are evaluated for thedetermination of the centric position. One possibility for theevaluation is an averaging from the coordinates of all points 21 whichlie in the hit field. The mean value is indicated by the cross 22 andillustrates the centric position. On the basis of knowledge of thecentric position 22 the dentist can now, by guiding of the lower jaw,cause the patient to move the lower jaw in a new single biting movementinto the bite end position indicated by the centric position and toprogram a register placed between the teeth. The thus programmedregister 9 is then placed between the teeth of the two plaster models inthe articulator 4 (FIG. 1). After that the joints 4, 5 of thearticulator 4 are so positioned in known manner that the teeth of theplaster model 7, 8 engage into the corresponding imprints in theregister 9 in form-fitting manner.

In addition or instead of the signal data of the determined centricposition also the jaw joint positions of the patient can be determined.This is shown in FIG. 5. The screen 20 shows here a coordinate system 20a in reduced form, likewise with points corresponding to a plurality ofdifferent bite end positions and the centric position 22 determinedtherefrom. Besides the representation 20 a, there are represented on thescreen 20, respectively on the left and on the right, two polarcoordinate planes 23, 24; 25, 26 which represent sections through thehead of the patient. The sections are designated by medical terms(dorsal, caudal, ventral, cranial, lateral and medial) and by thestylized head symbols located beside the polar coordinate planes. Thepolar coordinate planes 23-26 have the form of concentric rings. Thecentric position 22 in the Cartesian coordinate plane 20 a shown in thecentric position 22 corresponds to the polar coordinate points in thefour polar coordinate planes 23-26. The plurality of bite end positionsyield also in the polar coordinate planes a corresponding number ofpoints and through the evaluation of their positions a centric positionin form of a cross.

Instead—as was explained in connection with FIG. 4—of now using thedetermined centric position to program a register, it is possible todirectly use—i.e. without the intermediary of a register—the centricpositions of the jaw joints set down in FIG. 5 in the polar coordinateplanes, for adjustment of the joints of the articulator 1. For thisthere should be provided on the articulator a sender-sensor system whichcorresponds to that in FIG. 2. In this case the person carrying out theadjustment of the jaw joints can orientate himself on the display. As analternative to this it is however also possible that the joints of thearticulator are adjustable in motorized manner in three dimensions sothat the adjustment of the joint positions of the articulator need notbe effect by hand but is carried out automatically.

A third method for the determination of the centric position is shown inFIG. 6. Here the display 20 is again shown with a Cartesian coordinatesystem. The dentist causes the patient to move the lower jaw, with teethlying on each other completely or largely, to the front and to the rearas well as to the two sides. In this way a T-like movement pattern oflines appears on the screen, wherein the lines 27 represent the forwardand backward movement of the lower jaw and the lines 28 represent theside movements of the lower jaw. The upper limb of the T hangs down,i.e. is pointed or arched. The determination of the centric position iseffected by the evaluation by all movement lines, for example byaveraging, wherein the intersection point between the two T-limbs isdetermined as centric position 29 which is represented here again as across. In turn this centric position can be used for the programming ofa register or, in connection with the polar coordinates shown in FIG. 5,used for direct adjustment of the articulator.

1. Method for the determination of determining the centric position of ahuman set of teeth, comprising carrying out a plurality of bitingmovements having bite end positions of a patient's lower jaw, detectingthe biting movements with measurement technology, converting the bitingmovements and bite end positions into electrical signal data, anddetermining the centric position from the signal data of a plurality ofbite end positions.
 2. Method according to claim 1, comprisingdetermining the centric position by averaging the signal data of theplurality of bite end positions.
 3. Method according to claim 1,comprising evaluating only the signal data items lying in a coordinateplane in a restricted hit field for the determinations of the centricposition.
 4. Method for determining the centric position of a human setof teeth, comprising causing a patient's lower jaw to carry out at leastone movement with respect to the patient's upper jaw and evaluating atleast two movements to determine the centric position, furthercomprising causing the lower jaw in the bite end position to carry out aplurality of first displacement movements running between forward andrearward end positions as well as a plurality of second displacementmovements running between lateral end positions, detecting the first andsecond displacement movements with measurement technology, convertingthe first and second displacement movements into electrical signal data,and evaluating the signal data to the effect that an intersection pointbetween the first and the second displacement movements yields thecentric position.
 5. Method according to claim 4, comprising determiningthe signal data for the jaw joint positions, and determining the centricposition for each of the two jaw joints from the signal data for the jawjoint positions.
 6. Method according to claim 4, comprising representingthe signal data of the first and second displacement movement as well asthe signal data of the determined centric position on a display assymbols.
 7. Method according to claim 6, comprising representing thesymbols on the display in a coordinate plane.
 8. Method according toclaim 7, wherein the coordinates are Cartesian coordinates.
 9. Methodaccording to claim 4, comprising after determining the centric positionon the basis of the determined signal data, moving the lower jaw in acontrolled manner into the centric position, so that the teeth of theupper and lower jaws produce imprints, placing a plastic registerbetween the teeth of the upper and lower jaws, which register containsthe information about the centric position and the corresponding jawjoint positions.
 10. Method according to claim 1, comprisingrepresenting the signal data of the bite end positions of all bitingmovements and the signal data of the determined centric position on adisplay as symbols.
 11. Method according to claim 10, comprisingrepresenting the symbols on the display in a coordinate plane whichallows the actual spacings of the bite end positions from the centricposition to be recognized in appropriate measurement units.
 12. Methodaccording to claim 11, wherein the coordinates are Cartesiancoordinates.
 13. Method according to claim 5, comprising representingthe signal data of the centric position of the jaw joints on a displayas symbols, in each case in two coordinate planes, and the coordinateplanes include an angle.
 14. Method according to claim 13, wherein onecoordinate plane represents a sectional plane through the patient headwhich on the one hand runs between above (cranial) and below (cardial)and on the other hand between the front (ventral) and the rear(dorsally), and the other coordinate plane represents a sectional planethrough the patient head which on the one hand runs between the front(ventral) and the rear (dorsal) and on the other hand between the bodyaxis (medial) and the corresponding side (lateral).
 15. Method accordingto claim 13, comprising representing the coordinates in the manner ofpolar coordinates in the form of concentric rings.
 16. Method accordingto claim 9, comprising placing the register with the imprints into anarticulator in order, for the purpose of preparation of dentures, toalign models of the upper jaw and the lower jaw of the patients in thearticulator to one another.
 17. Method according to claim 5, comprisingusing the signal data of the centric position of the jaw joints for thedirect adjustment of the joints of the articulator or for the control ofmotor drives cooperating with the articulator.
 18. Device for carryingout a method according to claim 1 for determining the centric positionof a human set of teeth, comprising carrier frames for transmitter andreceiver elements, wherein in each case one carrier frame is connectablein a reproducible position with lower jaw and an upper jaw of thepatient concerned, and wherein the distances between the transmitter andreceiver elements are measured and evaluated for the extraction ofsignal data, wherein more than three transmitter elements and more thanfour receiver elements are provided.